1. Field of the Invention
This invention relates to a method of preparing chlorine dioxide in an electrolytic flow reactor. In particular, it relates to a method of preparing a stable solution of chlorine dioxide which will not disproportionate into chlorate and chlorite ions
2. Description of the Related Art
Early attempts by Logan (U.S. Pat. No. 2,163,793) to produce chlorine dioxide (ClO.sub.2) by the electrolysis of sodium chlorite solution containing sodium chloride gave a 72% conversion of the chlorite to chlorine dioxide. However, the solution leaving the electrolytic cell had a pH of 11-12 due to the following reactions: EQU Na.sup.+ +e.sup.-.fwdarw.Na EQU Na+H.sub.2 O.fwdarw.NaOH+.sub.2
The sodium hydroxide was the cause of the very high pH, which in turn caused the rapid decrease in ClO.sub.2 concentration due to the following rapid, irreversible, disproportionation reaction: EQU 2ClO.sub.2 +2OH.sup.-.fwdarw.ClO.sub.3.sup.- +ClO.sub.2.sup.- H.sub.2 O
Hence, if one intended to use the solution resulting from the above electrolysis as a deodorizer or disinfectant, the solution is practically useless due to the rapid loss of ClO.sub.2 concentration. Also, as shown in the above equation, the hydroxyl ion concentration tends to decrease thereby gradually lowering the pH of the disproportionating solution.
This deficiency has been overcome in various patents by using a semi-permeable membrane between the anode and cathode. The theory of this approach has been elucidated by Kascur in U.S. Pat. No. 5,084,149, filed Apr. 4, 1991.
The semi-permeable membrane separates the effluents from each electrode. The chlorine dioxide produced at the anode has a pH of 2-3. The sodium hydroxide produced at the cathode has a pH of 12 -13. The chlorine dioxide stream from the anode is very stable at that pH and useful as a deodorant or disinfectant.
However, it is very expensive to generate ClO.sub.2 using this system. It is not cost effective for the small applications for which it was intended. This is due in part to the membrane cost and the membrane's sensitivity to divalent ions, the complicated design and cost of the cell and power supply, and the necessity to periodically dispose of the by-products, sodium hydroxide and hydrogen.
The industry had developed on-site chemical generators which use sodium chlorite and chlorine gas as reactants to produce chlorine dioxide by oxidation of the sodium chlorite. These generators are used in applications where large quantities of chlorine dioxide are required. However, the use of chlorine as a reactant has come into disfavor due to the hazards of storing large amounts of chlorine gas. It is possible to substitute a combination of sodium hypochlorite (NaOCl) and an acid as a feed for the on-site chemical generators. However, the cost of generating ClO.sub.2 is increased appreciably.
Another approach to the generation of chlorine dioxide is to react sodium chlorite (NaClO.sub.2) with acid such as hydrochloric acid. However, this system has the disadvantage of slow reaction kinetics and more importantly, a theoretical yield of only 80% due to the conversion of 1 mole of sodium chlorite to sodium chloride according to the following equation: EQU 5NaClO.sub.2 +4HCl.fwdarw.4ClO.sub.2 +5NaCl+2H.sub.2 O
Japanese Patent 1866 (1956) claims the electrolysis of an equal mixture by weight of sodium chlorite and sodium chloride with a copper or nickel cathode and a graphite anode. The overall equation for this reaction is: EQU 2NaClO.sub.2 +2NaCl+H.sub.2 O.fwdarw.2ClO.sub.2 +2NaOH+H.sub.2 30 2NaCl
The yield is 72%. The hydrogen, sodium chloride and sodium hydroxide by-products are in contact with the chlorine dioxide, which results in an almost instantaneous loss of the chlorine dioxide due to the disproportionation reaction shown above, which is caused by the high pH of the sodium hydroxide containing effluent which renders the stream useless as a disinfectant/deodorizer due to the rapid loss of chlorine dioxide.
Japanese Patent 2316 (1956) discloses an electrochemical reduction of chlorates to chlorine dioxide in four compartments separated by ion exchange membranes using a porous carbon anode and an iron cathode.
U.S. Pat. No. 4,248,681 (issued Feb. 3, 1981 to Sweeney) discloses an electrolytic process in two sealed compartments separated by a Nafion R membrane using salt as the feed. A mixture of chlorine and chlorine dioxide is produced. Pure chlorine dioxide is never produced. British Patent No.714,828 (published Sep. 1, 1954) describes the electrolysis of a chlorite solution containing a water-soluble salt of an inorganic oxy-acid other than sulfuric acid.
Electrolysis of a chlorite solution containing a water-soluble metal sulfate is disclosed in U.S. Pat. No. 2,717,237 (issued Sep. 6, 1955 to Rempelin).
Control of the concentration of sodium chlorite in the anolyte at 0.8-5% by means of a photometer is disclosed in U.S. Pat. No. 4,542,008 (issued Sep. 17, 1985 to Capuano). A permselective membrane and electrodes containing an electrochemically active coating are used.
A particularly complicated electrolytic system has been patented by Halox in U.S. Pat. Nos. 5,419,816, 5,609,742 and 5,705,050. These patents describe a system in which the anode and cathode are separated by cation exchange membrane. In addition, the space between the anode and the cation exchange membrane is packed with a cation exchange resin, which must be replaced when spent. It is obvious that this cell is complicated and costly to construct and maintenance costs are high due to the need to replace the cation exchange membrane and cation exchange resin. Also the unit has capacity limitations due to the packing and cell construction.